Distributed clock generator for semiconductor devices and related methods of operating semiconductor devices

ABSTRACT

A distributed clock generator for a semiconductor device. In one embodiment, the clock generator is not localized in one particular location on the semiconductor die and then distributed, but instead the clock generation itself is distributed throughout the integrated circuit. The clock generator itself is a CMOS phase shift oscillator which uses a series resistance and a capacitance to ground. Phase shift elements utilize the phase shift of distributed transmission lines around the integrated circuit die and are thus conveniently implemented using the series resistance and parallel capacitance of the transmission lines.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/093,861, filed Mar. 8, 2002 is now a U.S. Pat. No. 6,952,117, towhich priority is claimed and which is incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

This invention relates generally to semiconductor devices, and moreparticularly relates to semiconductor memory devices operable using aclock signal.

BACKGROUND OF THE INVENTION

The field of semiconductor memory devices is enormously active andrapidly developing. Various categories and sub-categories ofsemiconductor devices are known and commercially available. Theever-increasing popularity and ubiquity of computers and computer-baseddevices, both in the consumer and industrial realms, is such that thedemand for semiconductor memory devices of a variety of different typeswill continue to grow for the foreseeable future.

One of the more common categories of semiconductor memory devices usedtoday is the dynamic random access memory, or DRAM. Among the desirablecharacteristics of any DRAM are a high storage capacity per unit area ofsemiconductor die area, fast access speeds, low power consumption, andlow cost.

One approach that has been used to optimize the desirable properties ofDRAM has been to design such devices such that they are accessiblesynchronously. A synchronous DRAM typically requires anexternally-applied clocking signal, as well as other externally-appliedcontrol signals whose timing must bear certain predeterminedrelationships with the clock signal. Likewise, digital data is read fromand written to a synchronous memory device in a synchronous relationshipto the externally-applied clock signal. Synchronous DRAM technologieshave been under development for many years, and synchronous DRAM(frequently referred to as “SDRAM”) is used in a broad spectrum ofcommercial and industrial applications, including the personal computerindustry.

In typical implementations, the external clock signal CLK for a clockedsemiconductor device (memory device or otherwise) comprises a simple,periodic “square” wave, oscillating with reasonably uniform periodicitybetween a logical high voltage level (for example, 3.3V) and a logicallow level (typically 0V) with a duty cycle of 50% (meaning that thesignal is at a logical “high” level the same amount of time that it isat a logical “low” level during each complete clock cycle). In presentstate-of-the-art semiconductor devices, the clock signal may have afrequency on the order of hundreds of megahertz.

A synchronous (clocked) semiconductor device typically requires anexternal clock signal to be applied to a clock (CLK) input. In the priorart, it is most commonly the case that the inputted CLK signal isprovided to an internal, central clock generator circuit, which in turngenerates one or more internal clock signals that are routed to variousfunctional blocks located at various locations on the semiconductorsubstrate.

One consideration that must be addressed relative to the routing ofclock signals throughout a synchronous semiconductor device is thewell-know problem of clock skew. See, for example, K. Yip, “Clock TreeDistribution: Balance is Essential for a Deep Sub-Micron ASIC Design toFlourish,” IEEE Potentials, vol. 16, no. 2, pp. 11–14, April–May 1997.Those of ordinary skill in the art will understand that the issue ofclock skew relates to the inevitable variations in the arrival time ofclock signals at different locations on a semiconductor substrate, wherethe master clock signal is generated at a single location on the part.Such variations arise due to such factors as propagation delays,variations in device switching speeds, and the like. Clock skew is afunction of two main parameters: the loading presented to the logicbeing clocked and the RC (resistive-capacitive) delay of the clock-lineinterconnect. Interconnect factors that affect clock skew include theresistance, capacitance, and inductance of the interconnectingconductors (“wires”), which typically comprise metal or otherwiseconductive traces formed as part of the semiconductor fabricationprocess.

Conductive wires in an integrated circuit (IC) are not ideal conductors,and differing lengths of wires in an IC can result in different delaysin the propagation of clock signals throughout the IC. As is widelyrecognized, clock skew adds to the effective clock cycle time for agiven semiconductor device, and hence adversely affects the performanceof the device.

For circuit designers, a typical rule-of-thumb regarding clock skew isthat it should be limited to ten percent or less of a chip's clockcycle, meaning that for a 100 Mhz clock, skew must be 1 nsec or less foreach clock signal in the device. High-performance semiconductor devicesmay require that the clock skew be limited to 5% or less of the clockcycle; for a 500 Mhz clock this would require skew to be 100 picosecondsor less.

As noted, prior art designs have concentrated on having one centralclock generator from which clock signals are distributed to functionalblocks throughout the devices. Different strategies have been employedfor clock signal distribution to avoid adverse consequences of clockskew. Perhaps the most common approach is to lay out the device suchthat all clock connections are symmetrical and of the same length. Anexample of this is the “H-tree” clock distribution scheme shown in FIG.1 a, wherein the conductive trace 12 representing the conductive tracesfor a clock signal are shown on a semiconductor substrate 10. An H-treeclock distribution strategy is used mostly in custom layouts and canfurther involve varying tree interconnect segment widths to balance skewthroughout the IC.

Another known clock distribution strategy is the grid layout, such asthe clock signal conductor 14 shown on substrate 10 in FIG. 1 b. A clockgrid is perhaps the simplest clock distribution scheme and has theadvantage of being relatively easy to design for low skew. However, asthose of ordinary skill in the art will appreciate, a clock grid ishighly inefficient in terms of occupation of area on the substrate 10and very “power hungry” due to the large number of clock interconnectsrequired. Nevertheless, some manufacturers do use this approach, inparticular, for microprocessors.

For high-performance semiconductor devices, a balanced tree distributionnetwork 16 is often employed. For a balanced tree without buffers, theclock line capacitance increases exponentially starting from the leafcell (i.e., a clocked element, an exemplary one being designated withreference numeral 18 in FIG. 1 c) and moving toward the clock input atthe root 20 of the tree. This extra capacitance results from theincreasingly wider metal traces needed to carry current to the branchingsegments. The extra metal required further results in additional chiparea to accommodate the extra clock-line width.

Buffers may be added at the branching points of a balanced tree clocknetwork, and this can have the effect of significantly reduced clockinterconnect capacitance, since it reduces clock-line width toward theroot. An example of a tree clock network 22 with buffers 24 is shown inFIG. 1 d. However, the buffers 24 undesirably occupy additional area onthe substrate and increase circuit complexity.

Those of ordinary skill in the art will appreciate that factors thatcontribute to clock skew include loading mismatch at the clockedelements, mismatch in RC delay due to variations in segment width andsegment length among the clock line segments, and process variationsinduced during chip fabrication. Inductance effects start to appear asclock-edge times and interconnect resistance decrease, both of whichoccurring more often with shrinking chip technology and higher clockrates. Clock trees often require wide traces at their roots and may alsohave long segments, making the trees more susceptible to inductanceproblems than other clock network schemes.

Careful layout, including placing power and ground lines next to, above,or below clock trees to act as shields, and can help reduce thepossibility of clock problems caused by inductance. Many designers andparasitic extraction/evaluation tools presently available address onlyRC parasitic effects. IC designers have heretofore not commonlyconsidered parasitic inductances, although this is more frequentlyconsidered as clock frequencies on state-of-the-art semiconductors areapproaching (or exceeding) 1 Ghz.

A different approach to addressing the problem with clock skew isreferred to as “delay-locked loop” or “DLL.” Various examples of DLLimplementations for synchronous memory devices are proposed in U.S. Pat.No. 5,920,518 to Harrison et al., entitled “Synchronous Clock GeneratorIncluding Delay-Locked Loop;” U.S. Pat. No. 6,201,424B1 to Harrison,entitled “Synchronous Clock Generator including a Delay-Locked LoopSignal-Loss Detector;” and U.S. Pat. No. 6,130,856 to McLaury, entitled“Method and Apparatus for Multiple Latency Synchronous Dynamic RandomAccess Memory.” The aforementioned '518, '424, and '856 patents are eachcommonly assigned to the Assignee of the present invention and each arehereby incorporated by reference herein in their respective entireties.

The function of a DLL circuit in a semiconductor device is to adjust therelative timing of clock signals provided to functional elementsdisposed at various locations on a semiconductor die such that overallsynchronous operation of the device can be achieved. DLL implementationsmay utilize some type of loop-back operation whereby the DLL circuit isprovided with feedback for comparing the timing of clock signalsprovided on various lines and provided to various functional elements ofthe device. As a result of the functionality of a typical DLL circuit,if the propagation and loading characteristice of one clock signaltransmission line vary significantly from others, the DLL circuit canaccount for such differences in order to ensure that proper deviceoperation can be maintained. Separate delays and skews (programmable orautomatically adjusted) may be introduced into the externally-appliedclock signal to ensure that each of the functional blocks in the devicereceives a clock signal that is substantially synchronized with theothers. Such delays and skews may be miniscule, on the order ofpicoseconds, but may be nonetheless critical to proper operation of asemiconductor device.

Another approach to addressing the problem of clock skew involves usinglow-impedance lines with matched terminations and current mode signalingto achieve well-defined, and hence more readily compensated-for, delays.See, for example, T. Knight et al., “Method for Skew-Free Distributionof Digital Signals Using Matched Variable Delay Lines,” Symposium onVLSI Circuits, Kyoto, Japan, May 19–21, pp. 19–20 (1993); see also, S.I. Liu et al., “Low Power Clock Deskew Buffer for High Speed DigitalCircuits,” IEEE Journal of Solid-State Circuits, v. 34, no. 4, pp.554–558 (1999).

Despite the various approaches proposed in the prior art, clock skewremains an ongoing challenge to integrated circuit designers, and it isbelieved that it is becoming increasingly important to address the issueof clock skew as device geometries shrink and system clock speeds rise.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus and corresponding methodfor providing a clock signal to clocked logic elements locatedthroughout an integrated circuit. In one embodiment of the invention, adistributed clock generator comprising a network of amplifier and phaseshift elements, including field-effect transistors and a diodesfunctioning as low-gain amplifiers, and series resistances andcapacitors to a ground plane functioning as phase shift elements, isdisclosed. In one embodiment, the phase shift elements are implementedas segments of an interconnection line or transmission lineinterconnecting the distributed amplifiers. The transmission linesegments may be configured as elongate metal interconnection lines eachdisposed in a spaced-apart relationship some distance above a groundplane of a semiconductor device so as to serve as a capacitance. Theaggregate length of all transmission line segments is chosen to achievethe desired total amount of phase shift, and the transmission linesegments are configured to form a closed loop extending throughout thesurface area of the semiconductor die so as to provide a clock signal tovarious locations on the die.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and aspects of the present inventionwill be best understood with reference to the following detaileddescription of a specific embodiment of the invention, when read inconjunction with the accompanying drawings, wherein:

FIGS. 1 a–1 d depict alternative clock signal wiring networkconfigurations for a semiconductor device;

FIG. 2 is a schematic diagram of a distributed phase-shift clockgeneration system in accordance with one embodiment of the invention;

FIG. 3 a is a schematic diagram of a phase shift element in theclock-generation system of FIG. 2;

FIG. 3 b is a cross-sectional depiction of a physical implementation ofthe phase shift element of FIG. 3 a;

FIGS. 4 a and 4 b are plots of the magnitude and angle, respectively ofthe transfer function of the phase shift element of FIGS. 3 a and 3 b;

FIG. 5 is a schematic/block diagram of an integrated circuitincorporating a distributed phase-shift clock generation system inaccordance with one embodiment of the invention; and

FIGS. 6 a and 6 b are plots of the magnitude and angle, respectively, ofthe transfer function of the phase shift element of FIGS. 3 a and 3 btaking into account inductive effects.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

In the disclosure that follows, in the interest of clarity, not allfeatures of actual implementations are described. It will of course beappreciated that in the development of any such actual implementation,as in any such project, numerous engineering and programming decisionsmust be made to achieve the developers' specific goals and subgoals(e.g., compliance with system and technical constraints), which willvary from one implementation to another. Moreover, attention willnecessarily be paid to proper engineering practices for the environmentin question. It will be appreciated that such a development effort mightbe complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the relevant fields.

Referring to FIG. 2, there is shown a distributed clock generatorcircuit 30 in accordance with one embodiment of the invention. As shownin FIG. 2, clock generator circuit 30 comprises a plurality ofamplifier/phase shift elements 32. Each amplifier/phase shift element 32comprises an n-channel field-effect transistor (FET) 34 and a diode 36which together function essentially as a low-gain amplifier. Eachamplifier/phase shift element 32 further comprises a series resistance38 and a capacitor 40. Optionally, each amplifier/phase shift element 32may further comprise a gate bias supply resistance 41 coupled betweenthe gate of respective transistor 34 and a gate bias voltage VGG. In oneembodiment, diodes 36 are implemented as diode-connected PMOStransistors.

Distributed clock generator circuit 30 can comprise an arbitrary numbern of amplifier/phase shift elements 32. Those of ordinary skill in theart will appreciate that for each amplifier/phase shift element 32, theRC phase shifting network comprising resistor 38 and capacitor 40introduces a phase shift in the signal amplified by the amplifiercomprising FET 34 and diode 36. For a given implementation of theinvention having an odd number n amplifier/phase shift elements 32, itis necessary to have the sum total of the phase shift across allamplifier/phase shift elements 32 equal to 180° (or some even integermultiple thereof) for proper operation of circuit 30. On the other hand,for an implementation having an even number n of amplifier/phase shiftelements 32, the sum total of phase shift across all elements 32 mustequal 360° (or some integer multiple thereof) for proper operation.

In one embodiment of the invention, the individual phase shift networkscomprising resistors 38 and capacitors 40 are implemented as segments ofan interconnection line or transmission line interconnecting theindividual amplifier/phase shift elements 32. FIG. 3 a is a schematicdiagram of a single amplifier/phase shift element from the embodiment ofFIG. 2 in isolation. As can be seen in FIG. 3 a, in addition toresistance 38 and capacitance 40, amplifier/shift element also has aparasitic inductive component represented in the schematic of FIG. 3 aby inductor 42. As is discussed hereinbelow, the effects of parasiticinductor 42 are primarily relevant only at extremely high frequencies,and for the purposes of the present disclosure can be more or lessignored.

FIG. 3 b is an end view of a phase-shift element 44 (at the same timerepresenting both resistance 38 and capacitance 40) implemented inaccordance with one embodiment of the invention as a substantiallyelongate metallic interconnection line between two amplifier/phase shiftelements 32 in circuit 30. For the purposes of the present disclosure,the physical geometry of phase-shift element 44 is approximated assubstantially cylindrical, although those of ordinary skill in the artwill appreciate that processing practicalities will likely lead to aslightly different geometry; nonetheless, the approximation issufficient for the purposes of the present disclosure and appreciationof the principles of the present invention.

With reference to FIG. 3 b, phase shift element 44 is disposed in aspaced apart relationship a distance d above a ground plane 46 of asemiconductor device. Ground plane 46 may be a semiconductor substrateor a metal grounding layer formed atop a substrate in accordance withconventional semiconductor design and fabrication techniques. In thepresently disclosed embodiment, phase shift element 44 has a height dsubstantially the same as the spacing between element 44 and the groundplane 46. Not shown in FIG. 3 b is an oxide layer or other dielectriclayer upon which phase shift element 44 is disposed to maintain thespacing d between phase shift element 44 and ground plane 46. The“radius” of phase shift element 44 (as noted above, phase shift element44 may not be purely cylindrical) is a.

Those of ordinary skill in the art will appreciate that the capacitanceof a cylinder over a plane is approximately calculated according to thefollowing formula:

$C = \frac{\pi ɛ}{\cosh^{- 1}\left( {h/a} \right)}$

(Equation 1), where ε is the permittivity of the dielectric materialbetween ground plane 46 and phase shift element 44, h is the distancebetween ground plane 46 and the central axis of phase shift element 44,and a is the radius of phase shift element a (again, assuming that phaseshift element 44 is cylindrical is an approximation made for thepurposes of simplifying the analysis).

In the presently disclosed illustrative example, it is assumed thatdimension d (the spacing between phase shift element 44 and ground plane46, as well as the height of phase shift element 44) is 1 micron, suchthat the dimension h is 1.5 microns. It is further assumed that thedielectric between phase shift element 44 and ground plane 46 is anoxide with a permittivity of 0.38×10–12 F/cm. In this exemplaryembodiment, therefore, the capacitance of phase shift element 44 can becalculated to be approximately 600 femto Farads per centimeter (fF/cm).The resistance is assumed to be on the order of 3×102Ω/cm, such that theRC value for each phase shift element is approximately 2.0×10–10fFΩ/cm2.

As noted above, those of ordinary skill in the art will appreciate thatthe phase shift elements 44 will also have some significant inductance;the effects of this inductance shall be described hereinbelow, and itwill be apparent to those of ordinary skill in the art will appreciatethat the inductive effects require a correction to the foregoingcalculations but will not change the general concept of the invention orsubstantially change the operation of a distributed phase shiftoscillator in accordance with the disclosed embodiment(s). In oneembodiment, the inductance of the phase shift elements 32 is estimatedto be on the order of 22 nH/cm.

Referring again to FIG. 3 a, those of ordinary skill in the art willappreciate that the transfer function H of phase shift element 44(ignoring, for the moment, inductive effects) can be approximated by:

${H \cong \frac{\frac{1}{j\;\omega\; C}}{R + \frac{1}{j\;\omega\; C}}} = \frac{1}{1 + {j\;\omega\;{RC}}}$

(Equation 2). Therefore, the angle of the transfer function H, i.e., ∠H,is given by:∠H=tan⁻¹ (ωRC)

(Equation 3). If ωRC is sufficiently small, the angle of the transferfunction H can be approximated by:∠H≅ωRC=ƒ(2πRC)

(Equation 4). With the resistance and capacitance as calculated above,this resolves to:∠H≅ƒ×(720×10⁻¹⁰ deg/cm)

(Equation 5). FIG. 4 a is a plot of the magnitude of the transferfunction H, i.e., |H|, and FIG. 4 b is a plot of the angle of thetransfer function H, i.e., ∠H. Consider an embodiment of the inventionin which the operating frequency f of the distributed clock signal is1.0 GHz. Consider further that an odd number, eleven, of phase shiftelements 44 are used, such that a total of 180° of phase shift isrequired. With the RC value as calculated above, and using Equation 5above, this means that a total length of 2.5 centimeters of phase shiftelements is required to achieve a total of 180° of phase shift. If eachof the eleven phase shift elements were of equal length, each elementwould contribute approximately 16° of phase shift. It is to beunderstood, however, that the present invention is not limited toimplementations in which each phase shift transmission line 44 is thesame length as the others. Indeed, depending upon the particularimplementation, it may be desirable to provide phase shift elements 44of various lengths. Those of ordinary skill in the art will appreciatethat it is the aggregate length of all transmission lines 44 that isdeterminative of the overall amount of phase shift achieved.

Turning to FIG. 5, there is shown the physical layout of an integratedcircuit 50 employing a distributed phase shift clock generating systemin accordance with one embodiment of the invention. As shown in FIG. 5,the clock generator comprises a plurality of interconnection ortransmission lines 44, which in one embodiment consist of 1 μm aluminumlines over 1 μm of oxide, as previously described with reference to FIG.3 b. The clock generation/distribution system further comprises aplurality of low-gain amplifiers 52 each consisting of a field-effecttransistor (FET) having a source terminal connected via a diode to apositive voltage supply (Vss) and a drain terminal coupled to ground.The interconnection lines 44 connect the amplifiers 52 in such a manneras to define a continuous, closed loop.

With continued reference to FIG. 5, each low-gain amplifier 52 may notonly be used to drive the next transmission line segment 44, but alsodrive buffers (not shown) providing clock signals (CLK) to differentareas and functional blocks on the integrated circuit 50, the physicaledge thereof being designated with reference numeral 54 in FIG. 5. Inaccordance with one aspect of the invention, a precise signal with awell-known phase relationship is thereby provided at many differentlocations on the integrated circuit die. Each buffer is also preferablycapable of serving to adjust the delay of clock signals (CLK) asnecessary and appropriate for each functional block. Those of ordinaryskill in the art will appreciate that the distributed phase shiftoscillator arrangement shown in FIG. 5 provides a stable clock signalwith precisely determined frequency and known phase relationship ordelay at each location. This is in contrast to prior art clockgeneration schemes, in which a clock signal generated at one location isdistributed around the area of the integrated circuit die.

As noted, the foregoing calculations generally ignore the inductiveeffects of individual phase-shifting line segments 44. That is, theeffects of inductor 42 in the model depicted schematically in FIG. 3 awere not entirely taken into account. As noted, on a short transmissionline segment 44, the inductive effects can be expected to result inslightly larger phase shift than if the line segment is just representedby resistive and capacitive elements, as in the foregoing equations. Theline segments can be made slightly shorter than the RC computationsresolve to account for this approximation. These corrections do noteffect the concept of operation, but rather are second-order effectswhich can be included in precise models and calculations.

In particular, the transfer function H for phase shift element 32including inductive considerations can be expressed as:

$\begin{matrix}{H = \frac{\frac{1}{{j\;\omega\; C}\;}}{R + {j\;\omega\; L} + \frac{1}{{j\;\omega\; C}\;}}} \\{= \frac{1}{{\left( {j\;\omega} \right)^{2}{LC}} + {j\;\omega\;{RC}} + 1}} \\{= {\left( \frac{1}{LC} \right)\left( \frac{1}{\left( {j\;\omega} \right)^{2} + {j\;{\omega\left( {R/L} \right)}} + \frac{1}{LC}} \right)}}\end{matrix}$

(Equation 6), where the magnitude |H| of the transfer function is givenby:

${H} = {\left( \frac{1}{LC} \right)\left( \frac{1}{\sqrt{\left( {\left( \frac{1}{LC} \right) - \omega^{2}} \right)^{2} + \left( \frac{\omega\; R}{L} \right)^{2}}} \right)}$

(Equation 7), and the angle ∠H of the transfer function is given by:

${\angle\; H} = {\tan^{- 1}\left( \frac{\left( \frac{\omega\; R}{L} \right)}{\frac{1}{LC} - \omega^{2}} \right)}$

(Equation 8). FIGS. 6 a and 6 b show exemplary plots of the magnitudeand angle of the transfer function H of the phase shift elements 44,respectively, in accordance with one embodiment of the invention. InFIGS. 6 a and 6 b, the transfer function as calculated without takinginto account the inductive effects is represented with dashed lines,while the transfer function as calculated including inductive effects isrepresented with solid lines. From FIG. 6 b, it is apparent that on ashort line segment 44 the inductive effects will result in a slightlylarger phase shift than if the line is just modeled by its resistive andcapacitive elements. The line segments can be made slightly shorter tocompensate for this second-order differential.

From the foregoing detailed description of specific embodiments of theinvention, it should be apparent that a distributed, phase shiftoscillator for a semiconductor device has been disclosed. Althoughspecific embodiments of the invention have been disclosed herein in somedetail, this has been done solely for the purposes of describing variousfeatures and aspects of the invention, and is not intended to belimiting with respect to the scope of the invention. For instance, theparticular dimensions and composition of phase shift elements 44disclosed herein are provided for illustrating the basic principles ofthe present invention, and it is by no means intended that the inventionbe limited to the specific dimensions and composition disclosed herein.Likewise, it is contemplated that the invention may be practiced inconnection with various categories of semiconductor devices requiring anon-chip clocking signal, including, without limitation, synchronousmemory devices such as the synchronous DRAM disclosed in U.S. Pat. No.6,327,196 to Mullarkey, entitled “Synchronous Memory Device Having anAdjustable Data Clocking Circuit,” which patent is commonly assigned tothe assignee of the present invention and hereby incorporated byreference herein in its entirety. It is contemplated that varioussubstitutions, alterations, and/or modifications, including but notlimited to those implementation variations which may have been suggestedherein, may be made to the disclosed embodiments without departing fromthe spirit and scope of the invention as defined by the appended claims,which follow.

1. An integrated circuit having a plurality of functional blocks atdifferent locations throughout a layout of the integrated circuit,comprising: a plurality of amplifiers distributed at a plurality ofspaced-apart locations throughout the layout of the integrated circuit;and a plurality of phase shift elements each coupled between two of theplurality of amplifiers so as to define a closed loop, wherein theclosed loop is tapped at a plurality of locations to form a plurality ofclock signals of different phases, wherein the plurality of clocksignals are each sent to various of the functional blocks.
 2. Theintegrated circuit of claim 1, wherein each of the plurality ofamplifiers comprises a transistor serially connected to a diode.
 3. Theintegrated circuit of claim 2, wherein a positive terminal of the diodeis couple to a voltage supply, wherein a negative terminal of the diodeis coupled to a drain of a transistor, wherein the source of thetransistor is coupled to ground, and wherein a phase shift element iscoupled to a gate of the transistor.
 4. The integrated circuit of claim1, wherein the plurality of phase shift elements comprises an odd numbercollectively introducing substantially 180°, or an even integer multipleof 180°, of phase shift.
 5. The integrated circuit of claim 1, whereineach of said plurality of phase shift elements comprises a discreteresistor and a capacitor.
 6. The integrated circuit of claim 1, whereineach of said plurality of phase shift elements comprises a conductivetransmission line having a resistance and a capacitance.
 7. Theintegrated circuit of claim 6, wherein each conductive transmission linecomprises an elongate conductor disposed in a spaced-apart relationshipwit a ground plane of the integrated circuit.
 8. The integrated circuitof claim 7, wherein the spaced-apart relationship is maintained by aninsulating layer disposed between the elongate conductor and the groundplane.
 9. The integrated circuit of claim 6, wherein at least some ofthe plurality of conductive transmission lines are of varying length.10. The integrated circuit of claim 6, wherein the aggregate length ofthe plurality conductive transmission lines is such that the total phaseshift provided by the conductive transmission lines is substantially180°, or an even integer multiple of 180 °.
 11. The integrated circuitof claim 6, wherein the aggregate length of the plurality conductivetransmission lines is such that the total phase shift provided by theconductive transmission lines is substantially 360°, or an integermultiple of 360 °.
 12. An integrated circuit having a plurality offunctional blocks at different locations throughout a layout of theintegrated circuit, comprising: a plurality of amplifiers distributed ata plurality of spaced-apart locations throughout the layout of theintegrated circuit; and a plurality of phase shift elements each coupledbetween two of the plurality of amplifiers so as to define a closedloop, wherein the closed loop is tapped at a plurality of locations toform a plurality of clock signals of different phases, wherein theplurality of clock signals are each sent to various of the functionalblocks, wherein the plurality of phase shift elements comprises an evennumber collectively introducing substantially 360°, or an integermultiple of 360°, of phase shift.
 13. The integrated circuit of claim12, wherein each of the plurality of amplifiers comprises a transistorserially connected to a diode.
 14. The integrated circuit of claim 13,wherein a positive terminal of the diode is couple to a voltage supply,wherein a negative terminal of the diode is coupled to a drain of atransistor, wherein the source of the transistor is coupled to ground,and wherein a phase shift element is coupled to a gate of thetransistor.
 15. The integrated circuit of claim 12, wherein each of saidplurality of phase shift elements comprises a discrete resistor and acapacitor.
 16. The integrated circuit of claim 12, wherein each of saidplurality of phase shift elements comprises a conductive transmissionline having a resistance and a capacitance.
 17. The integrated circuitof claim 16, wherein each conductive transmission line comprises anelongate conductor disposed in a spaced-apart relationship with a groundplane of the integrated circuit.
 18. The integrated circuit of claim 17,wherein the spaced-apart relationship is maintained by an insulatinglayer disposed between the elongate conductor and the ground plane. 19.The integrated circuit of claim 16, wherein at least some of theplurality of conductive transmission lines are of varying length.